1. Field of the Invention
This invention relates to locks and especially to locks which can be opened only through the use of two separate keys.
2. Discussion of Related Art
The workings of the standard tumbler lock are now well known and such a lock can be picked with relative ease by one who possesses the right tools and a minimum of skill. Accordingly, a need has arisen for a lock which is more difficult to pick yet relatively simple in construction and operation and inexpensive to manufacture.
Various locks have been suggested which are felt to met these requirements. For instance, U.S. Pat. No. 3,473,355, issued Oct. 21, 1969 to Matsugaoka, shows a two-key lock whose bolt can be opened and closed only by using two different keys in succession. The lock includes three concentric cylinders with pin tumblers arranged in the cylinders so that one of the keys is inserted into a keyway in the innermost cylinder, the two inner cylinders rotate together 180`, and when the first key is replaced by the second key, only the outermost cylinder will rotate 180.degree.. U.S. Pat. No. 3,916,657, issued Nov. 4, 1975 to Steinbach, shows a double key axial split pin tumbler-type lock. The Steinbach lock includes a three-part plug assembly inserted in the cylinder and a plurality of tumbler assemblies carried by the plug assembly. The plug assembly includes a rear sleeve part fixed to the cylinder, a rotatable intermediate spindle part including a lock shaft extending through the sleeve part, a rotatable front drive part, and a post fixed to the drive part having means drivingly engaging a key for rotation of the drive part by turning the key. U.S. Pat. No. 2,667,785, issued Feb. 2, 1954 to Opocensky shows a two-key cylinder lock comprising a cylindrical sleeve formed with a plurality of circumferentially spaced internal seats presenting transversely opposed abutment shoulders. A key plug is rotatably mounted in the sleeve and a plurality of spring biased tumblers are carried by the plug and have ends normally projected into the seats. One of the ends is shaped to interlock with only one of the shoulders and another of the ends is shaped to interlock with only the opposite shoulder.